A. Field Of The Invention
The present invention is directed to a water dispersible hot melt adhesive composition and to "Perfect Bound" books constructed using the adhesive.
The adhesive composition of the present invention gives excellent adhesion both to ground wood and newsprint stock, and to high quality coated stock and heavily inked paper. In addition, the water dispersible nature of the adhesive of the present invention permits the repulping of books and trimmings made using the adhesive.
B. Prior Art
In light of the increasing public concern for the conservation of limited natural resources and limits to the availability of landfill space, recycling of products derived from natural resources is of great interest and importance.
The recycling of paper and paper products represents an area of recycling which has met with some success but is not without its difficulties.
A general method for the recycling of paper is reported by the American Paper Institute in an article by R.S. Forsyth entitled "Recyclable/Repulpable Hot Melts--A Survey--USA And Europe--A Challenge," TAPPI Symposium (1989) In the disclosed method, paper is mixed with water and the fibers are separated via mechanical action similar to that of a kitchen blender. The slurry is then passed through screens and centrifugal cleaners to remove non-fibrous contaminants such as glass, metal, plastic, dirt, and other unwanted solid materials. After waste paper is repulped, it is formed into paper or paper board on the moving wire of a fourdrinier machine or on the cylinders of a cylinder machine, pressed and dried to remove the remaining water.
One of the difficulties inherent in the recycling of certain paper products is the fact that many of them contain water insoluble hot melt and pressure sensitive adhesives. These adhesives are considered the most difficult contaminants to remove from pulp fiber during the repulping process The failure to remove all of the water insoluble adhesive from the repulped paper yield papers of low quality, inconsistent composition, non-uniform appearance and non-uniform surfaces. Additionally, the sticky nature of the adhesives creates processing difficulties in the automated machinery in common use in the paper industry. The inability to effectively remove these synthetic contaminants stands as one of the major barriers to the increased recycling of paper waste.
One solution to the problem was offered by Ray-Chandhuri (U.S. Pat. No. 3,891,584) which discloses a water dispersible hot melt adhesive comprising 75-95 parts of a graft copolymer of vinyl monomer and a water soluble polyalkylene oxide polymer, and 25-5 parts of tackifying resin. A problem with the method of the '584 patent is that it is cumbersome to run, requiring reflux under a nitrogen atmosphere for 2 hours and vacuum distillation to remove residual monomer.
At present, the only hot melt adhesive for bookbinding that we know of that is both commercially viable and also water dispersible are the "animal glues". "Animal glues" are gelatin adhesives made from the hide and bone of animals. Because the animal glues are water soluble, the bookbinder, who uses them, is able to combine his book trimmings with his other paper waste and sell them for the purpose of repulping.
The problems with "animal glues" in bookbinding are twofold. First, they don't adhere well to stocks with coatings or heavy ink coverages. Secondly, they require very specialized application pots due to their high viscosities. The biggest application for animal glue is in telephone directory binding. This is because directories use predominantly ground-wood or news print paper and not higher quality stocks with heavy ink coverages and with coatings (e.g., enamels, lacquers, etc.). However, over the past few years, the U.S. publishers of directories have become more advertising conscious and have required that the higher quality papers also be bound. The demand for these higher quality papers has forced the printers and binders of directories away from the animal glues and toward the EVA and rubber-based hot melt adhesives in ever increasing amounts. However, as already pointed out, the EVA and rubber-based hot melt adhesives, which were not water dispersible, prohibit the binder from repulping his trimmings and prevent the book ultimately from being readily recycled
It is an object of the present invention to formulate a water dispersible adhesive composition that is capable of effectively binding both to ground-wood or news print paper and to paper stocks with coatings and/or heavy ink coverages.
Telephone directories, particularly directories located at outdoor telephones, present a unique variety of problems These directories occasionally hang in a phone booth with their back (binding side) up and the pages hanging down. When hanging in this fashion, the pages are subject to the constant downward pull of gravity under a variety of weather conditions, including heat and high humidity. The combined conditions of heat and high humidity are particularly adverse for a water dispersible hot melt adhesive composition. It is an object of the present invention to provide a water dispersible hot melt adhesive composition that is capable of functioning reasonably well under conditions of heat and high humidity.